1. Field of the Invention
This invention relates generally to fasteners and, in particular, to a self-aligning dome nut.
2. Description of the Prior Art
Self-aligning nuts are useful where precise alignment of the cooperating male member is either difficult to obtain or too laborious. For example, with mass produced items where a number of repetitious connections are required to fasten two members together, production standards and efficiency can be improved with pre-assembly of the necessary nuts to one of the members. Where the nuts are fixed in position on the one member, mating of the other member and the cooperating male fasteners must be precise; and this results in a time-consuming assembly operation. To avoid this, nut constructions have been devised where the nut member actually floats in its connection to the one member. In this way, it can adjust itself to the proper position necessary for connection of the male member even when the male member is slightly out of alignment. The assembly operation is thus made easier and quicker.
One example of the prior art constructions includes a base member having a neck portion which is serrated on its outer periphery and adapted to be inserted into the hole of the mounting member. On the side of the base member opposite the neck portion, a threaded nut member is loosely attached whereby insertion of a threaded male member through the neck portion and into threaded engagement with the nut can be easily obtained without precise alignment of the nut. A nut of this construction is disclosed in U.S. Pat. No. 2,986,188.
In order to effect attachment of the above-described nut into the hole of the mounting member, a suitable attaching tool having a threaded end may be inserted through the neck portion of the base member and into threaded engagement with the floating nut. Tightening of the tool member by threading further into the nut will draw the neck into the hole of the mounting plate to effect attachment. Nut structures of this type are adequate where the clamping force required to effect attachment to the mounting member is not too severe so as to cause stripping of the threads during use of the attachment tool. Also, prior art structures of this type are suitable only where a fluid tight seal through the nut member is not required.
Other prior art constructions have been devised to provide for a fluid tight connection of the nut structure to the mounting member. One such structure is disclosed in the abovementioned patent and includes a base member having a neck portion on one side for insertion into the hole of the support member and a nut positioned on the other side of the base member. The nut is loosely or floatingly held in place by a protective cover which is crimped over the base member. To effect sealing, a suitable O-ring is positioned between the base member and the support member and another sealing member may be positioned between the base member and the protective cover. Constructions of this type can be connected to the support member with the threaded attaching tool described above, or alternatively, clamping forces can be applied to the protective cover pressing it against the support member to effect insertion of the neck into the hole of the support member and clinching thereof if desired.
As with the non-sealing type of nut, the use of a threaded attaching tool has limitations as to the force that may be applied in attaching the nut; and where clamping forces are applied to the protective cover, they can tend to disrupt the crimped connection of the cover to the base member and also distort the cover into the engagement with the floating nut. This, in turn, can eliminate the floating orientation of the nut and jam it so that it will not properly align with the threaded male member. This is particularly true where the forces required to effect a secure connection of the nut to the base member are relatively high.
Another prior art construction that provides for both self-alignment of the nut member and sealing of the structure to the support member incorportes a protective cover which is provided with two laterally extending flanges. Each of these flanges is provided with a hole through which rivets may be inserted for securing the cover and the enclosed floating nut to the support member. Structures of this type are disadvantageous in that that requires three holes in the support member, one for each of the rivets and a center hole for the insertion of the cooperating male member.
Nut constructions such as described above and to which the present invention relates, are used to facilitate attachment of the nutted support member to other structure. An example is in the aircraft industry where quick, accurate connection of the many sub-assemblies used in constructing the aircraft is desirable. Depending on the particular sub-assembly, it is sometimes necessary to electrically isolate the extending nuts from the surrounding structure. For example, with aircraft fuel tanks, the closure valve assembly through which fueling takes place is typically constructed as a sub-assembly and attached to the fuel tank wall be means of self-aligning nuts. These nuts protrude into the interior of the fuel tank and create a condition susceptible to causing electrical sparking or arcing. Electrical arcing may tend to occur as, for example, where the exterior of the fuel tank on the aircraft is struck by lightning and the electrical charge passed into the fuel tank by way of the attaching nuts. To preclude this electrical condition, it has been the past practice to coat each nut assembly with a sealing compound of non-conductive material. This operation, however, is a time-consuming and expensive hand operation. To be sure of proper electrical isolation, all edges of the nut must be carefully coated and the nut completely covered at its peripheral areas of contact with the interior wall surface of the tank.